The Curly Hazel or, as I would call it Harry Lauders Walking Stick is usualy sold as a grafted shrub ie the curly bit is grafted onto a rootstock of the common hedgerow hazel. What has happened with yours, Ponty, is that the rootstock has put up shoots and these now dominate the shrub, effectivly killing off the curls. This is not necessarily a bad thing if you like the hazel and, particulary, if it produces edible nuts!
I would expect it to be showing catkins by now in the south but the only way to control it will be by pruning whilst the shrub is dormant.
We have two of these shrubs. One must have been grown from a cutting as it has not produced the straight shoots. The other one does produce the shoots occaisionally and we cut them off as soon as we notice them.

It's as Bulbaholic says, ours are full of catkins, I am still cutting back the straight shoots for use in the veg. garden. Look for the little red flowers on the bushing out parts, they are the female flowers which will produce nuts.

Thanks, both.
Harry Lauder's Walking Stick! I remember that crooked stick although I was a child when he died. I did not realise it had any connection! Do I take it that it is OK to simply prune the higher growth? I assume, Bob, that you use the cuttings as pea sticks etc., I will check for the red flowers when the rain stops.

Yes, Ponty, use them for peas, beans or sweet peas.